Question
Explain co–dominance with the help of one example.
✓
Answer
Co–dominance:
- The alleles which are able to express themselves independently, even when present together are called co–dominant alleles and this biological phenomenon is called co–dominance.
- ABO blood groups are controlled by gene I. Gene I has three alleles IA, IB and $I^O/i.$
- $I^A$ and $I^B$ produce RBC surface antigens sugar polymer A and B, respectively, whereas i does not produce any antigen. $I^A$ and $I^B$ are dominant over I hence $I^A$ and $I^B$ are dominant alleles and I is recessive allele as in $I^ai$ and $I^bi.$
- When $I^A$ and $I^B$ are present together, both express equally and produce the surface antigens A and B, hence show co–dominance.
- Since humans are diploid, each person possesses any two of the three ‘I’ gene alleles, resulting into six different genotypic combinations and four phenotypic expressions.
|
Allele from Parent 1
|
Allele from Parent 2
|
Genotype of off spring
|
Blood groups of off spring
|
| $I^A$ | $I^A$ |
$I^AI^A$
|
A
|
| $I^A$ | $I^B$ | $I^AI^B$ |
AB
|
| $I^A$ |
i
|
$I^Ai$
|
A
|
| $I^B$ | $I^B$ | $I^BI^B$ |
B
|
| $I^B$ |
i
|
$I^Bi$
|
B
|
|
i
|
i
|
ii
|
O
|
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